Current transformers are electrical devices which, provided with an input (or primary) alternating current, generate an output (or secondary) alternating current which is a multiple or a fraction of the input current. Current transformers are traditionally used in electrical measurement and calibration applications where high precision is often a requirement. A current transformer used for electrical measurement generally has an input current in the range of the 100's of amperes (Amp) and an output in the range of 100's of miliamperes (mA.) A current transformer used for electrical measurement applications will have a known input/output current ratio which allows for the determination of the level of the input current from a relatively low, but proportional, output current.
For obvious reasons, it is desirable that the level of the input/output current ratio be as precise as possible throughout the range of expected input current levels. In order to achieve the level of precision required for high-end applications, however, current transformers of conventional designs have suffered from several shortcomings.
Specifically, a high-precision current transformer of traditional design must have very small magnetizing or excitation current (Imag) levels with an ideal transformer having an Imag of zero. Since Imag is directly proportional to the magnetizing voltage (Vmag), Vmag also must be relatively small. Both Imag and Vmag are considered parasitic losses and must be minimized in order to improve the accuracy of a high-precision current transformer. This, in turn, requires that the magnetizing inductance (Lmag) of the transformer be very large and conversely that loses in the transformer's core be relatively small.
In order to achieve the required large Lmag values, a common approach is to utilize a secondary winding that has a large number of turns of very fine wire material. This approach, however is limited in that as the number of turns increases, and the cross section of the wire decreases, the resistance of the secondary winding increases and creates an unwanted proportional increase in Vmag and Imag. This can be counteracted by utilizing heavier gauge wire and core materials having high electrical permeability and low loss factors. However, these measures tend to significantly increase the size and cost of the current transformer.
A second shortcoming of traditional current transformer designs is the fact that they are very intolerant to direct current (DC) components present in the input or primary current (Ipri). Because Lmag values must be maintained at very high levels to improve accuracy, and because the resistance of the secondary winding must be kept relatively low, the L/R time constant for this circuit tends to be so large that the DC component of Vmag is integrated to produce a DC component of Imag that is also very large and degrades the precision of the current sensed through the burden resistor.
Of course, this effect could be counteracted by reducing Lmag or increasing the secondary winding's resistance. However, this would, in turn, have a negative impact in that it would increase Imag from the AC component and ultimately deteriorate overall accuracy.
The way this shortcoming is normally addressed in traditional current transformer designs is simply by utilizing core materials which have a high flux tolerance and relatively low loss factors. The primary DC component will produce a corresponding DC component in the secondary. For a half wave rectified sine of primary current the DC bias in the secondary current can cause an additional RMS error up to 23%, As the portion of normal AC current increases the corresponding error decreases. As before, however, these measures would have a negative impact on the size and cost of the resulting transformer.
Previous attempts have been made to improve the performance of current transformers through the use of impedance compensation. Examples of such attempts are described in U.S. Pat. No. 3,534,247 to Miljanic (the '247 patent); U.S. Pat. No. 4,628,251 to Halder (the '251 patent); and U.S. Pat. No. 5,276,394 to Mayfield (the '394 patent).
However, none of these references describes a high-precision current transformer incorporating an impedance compensation circuit like the one described in detail below which continuously exposes the secondary winding of the transformer to virtual impedance of equal but negative magnitude to its own, thus virtually reducing the values of Imag and Vmag to zero.
Furthermore, none of the above references describes a high-precision current transformer incorporating an impedance compensation circuit that exhibits high tolerance to elevated direct current components in the input current.
Therefore, there is a need in the prior art to provide a high-precision current transformer incorporating an impedance compensation circuit which permits operation with magnetizing current and voltage levels at or near zero.
There is a further need in the art to provide a high-precision current transformer that does not require high levels of magnetizing inductance to maintain its accuracy.
There is a further need in the art to provide a high-precision current transformer that does not require core materials which have high electrical permeability or low loss factors.
There is a further need in the art to provide a high-precision current transformer that does not require the use of a relatively large secondary winding composed of lower resistance, heavier gauge, wire.
There is a further need in the art to provide a high-precision current transformer that is highly tolerant of elevated DC components in the input current without excessive degraded accuracy.
There is a further need in the art to provide a high-precision current transformer that increases the tolerance for elevated DC components in the input current without utilizing core materials which have a high flux tolerance or relatively low loss factors.